Abstract Mammalian microRNAs are critical regulators of several human diseases. As a result, therapeutic manipulation of miRNAs in diseased tissues has emerged as a promising approach to combating human disorders, and has had success in some settings. Yet, effective delivery of miRNAs or their inhibitors to many cell and tissue types continues to be a major challenge, and this underscores the need for improved methods of small RNA delivery to relevant cell types. Recent reports have demonstrated that miRNAs can be released from cells in small lipid vesicles called exosomes. In turn, the exosomes can deliver miRNAs to recipient cells, and this system is thought to constitute a novel form of intercellular communication. This process includes an endogenous miRNA delivery mechanism that is largely uncharacterized, yet could provide valuable insights into how therapeutic miRNA delivery can be improved and how inflammatory responses are regulated. Our preliminary data demonstrate that mouse bone marrow derived dendritic cells (BMDCs) produce exosomes that contain specific miRNAs, including the inflammatory regulators miR-155 and mIR-146a, that can be delivered to recipient immune cells and subsequently mediate target knockdown both in vitro and in vivo. This results in an altered response to endotoxin both in vitro and in vivo, which provides experimental evidence that exosome-transferred miRNAs provide a novel layer of regulating inflammation. Further, we have also been able to successfully load specific miRNA mimics into exosomes and demonstrate that they can be delivered to recipient cells in a functionally relevant manner. As a result of these findings, we hypothesize that exosomal miRNAs play novel regulatory roles during physiologically relevant inflammatory responses, and that through an improved understanding of this process, exosomes can ultimately be coopted to deliver specific miRNA cocktails in a therapeutically relevant manner. We will carry out the following specific aims to test these predictions. First, we will define the process of exosomal miRNA delivery to immune cells. Next, we will determine the functional relevance of exosomal miRNAs during inflammation using radiation chimeras and Rab27a/b DKO mice with impaired exosome production. Finally, we will engineer custom exosomes and test their impact on inflammatory disease. Together, this project will shed light on how exosome transfer of miRNAs is regulated, determine the role of this system during physiologically relevant inflammation, and begin to understand the translational potential of this novel process.